People want wide, this is the reason of curvatures.
In bigger photography there are rectilinear fish-eyes, which are wide but keep lines straight.
For this hobby niche seems nobody is interested to make such lens, so the only way to go is longer focal and narrower angles, under 90 degrees mandatory.
Below you have the lens formula for a rectilnear (first, left) and a circular (second, right) fish-eye.

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Considering the expense of manufacturing all those lens elements to the precision needed, the impetus to do that work for the cheap disposable action cameras is low.
The GoPro does a reasonable job for a video camera.. nice big image. No match for the high end 16Mpix name brand cameras for still images though.